Conventionally, in a monochromatic digital copier of electrophotography system, a reflected light from an illuminated color original is received by photoelectric conversion elements such as CCDs via color separation filters of red (R), green (G) and blue (B). In this way, color separation is applied to obtain light reception signals R, G, B in proportion to the light intensities of R-, G- and B-components. After analog-to-digital (A/D) conversion is applied to these signals to obtain digital values, density conversion is applied to obtain color image data of cyan (C), magenta (M), yellow (Y) in proportion to densities.
Subsequently, the monochromatic or color image data having the density conversion applied thereto are subjected to a variety of image processings including a black generation, a color correction, a magnifying processing, a spatial filtering and a gradation correction, thereby correcting the image data. An output is controlled using the corrected image data.
The above gradation correction is performed by correcting a gradation data in order to prevent a fogging and to obtain a desired or suitable density, and the following two methods (1), (2) are known for this correction.
(1) For example, in the case of a color copier, gradation data within a range determined by predetermined threshold value(s) are extracted as effective image data from a frequency distribution curve of the gradation data of all pixels corresponding to the light reception signal G representing a characteristic most approximate to lightness among the light reception signals R, G, B or to the image data M representing a characteristic most approximate to lightness among the image data C, M, Y, and the gradation correction is performed using the effective image data.
(2) Japanese Unexamined Patent Publication No. 7-184070 discloses a gradation correction for converting a gradation data of an inputted image by following equation (1) which is a linear conversion equation: EQU h(x)=255(x-.alpha.)/(.beta.-.alpha.) (1)
It should be noted that the image data is converted into a digital data of 8 bits by the A/D conversion and that .alpha., .beta., x and h(x) denote minimum and maximum values of the gradation data of the inputted image, a gradation data before a correction and a gradation data after the correction, respectively.
By performing this gradation correction, h(.alpha.)=0 when x=.alpha., h(.beta.)=255 when x=.beta., in other words, the gradation data of .alpha..ltoreq.x.ltoreq..beta. is uniformly converted into gradation data of 0.ltoreq.h(x).ltoreq.255.
However, color may change since the gradation data is obtained from the frequency distribution curve of the gradation data of all pixels corresponding only to the image data M representing a characteristic most approximate to lightness according to the prior art method (1).
On the other hand, according to the prior art method (2), equation (1) is a linear equation. As can be seen from equation (1), in the case that the data are identically distributed where (.beta.-.alpha.) is the same in the range .alpha..ltoreq.x.ltoreq..beta., the distribution of the gradation data after the correction is identical even if the values of .alpha., .beta. of the original image differ. Thus, if the gradation correction is performed by equation (1), information carried by the original image may be lost.
In view of the above problems residing in the prior art, an object of the present invention is to provide an image processing apparatus which can provide a satisfactory image corresponding to an original image by an improved gradation correction.